Oscillator circuits produce periodic electrical wave-forms, such as sine waves, at a frequency determined by intrinsic component values. Oscillators are widely used in electronic applications, especially in radio receivers, as sources of radio frequency energy. Radio receivers oscillators, called local oscillators, operate in the internal mixer circuits, which convert the incoming radio signals up or down to a fixed intermediate frequency to be amplified and detected and later turned into an audible signal. Typical oscillators that operate in high frequencies, such as in the radio frequency range, tend to generate unwanted electrical noise. This noise is harmonically related to the frequency of the local oscillator and can travel outward from the receiver to interfere with other nearby radios, cellular phones, citizen band radios, etc.
Reductions in these unwanted radiated emissions minimizes the chance of interference with other nearby devices, and, such reductions, may be required to meet radiated emissions limitations established by regulatory agencies both in the U.S. and abroad. Known methods to reduce radiated emissions include adding metal shielding and filtered connectors to the unit in which the oscillator is employed. In general, this is an expensive proposition due to a proliferation in hardware required.
FIG. 1 shows an ideal sine wave signal 12, having a singular spectral frequency 14, shown in FIG. 2. With the ideal sine wave shown in FIGS. 1 and 2, there are no unwanted harmonics that radiate as noise.
FIG. 3 illustrates a typical oscillator output as deformed sine wave 16. FIG. 4 shows the spectral plot of deformed sine wave 16, illustrated by plots 18 in the graph shown. Frequency f.sub.0 is the desired frequency of sine wave 16 and frequencies f.sub.1 -f.sub.n are the unwanted harmonics, which, in the absence of preventative measures, radiate from the oscillator as unwanted emissions.
Referring to FIG. 5, a typical prior art local oscillator circuit is shown. The frequency of the circuit is controlled by inductor 20, capacitor 22 and varactor 24, comprising the tank 21. The frequency of the oscillator is tuned by the voltage level set on line 49, which is coupled to the tuning input of varactor 24 by resistor 48 and capacitor 50 as shown.
Tank 21 is coupled to transistor 34 via capacitors 26 and 28. Transistor 34 has its base biased into the active region by resistors 30 and 32 and has its collector biased high by resistor 36 coupled to the power supply. Capacitor 42 filters current spikes from the power supply. The output of transistor 34 is fed back to the tank circuit via inductor 38 and resistor 40, as shown. The output signal at the collector of transistor 34 is filtered by inductor 44 and capacitor 46 and output for its desired use on line 45.
FIG. 6 shows the spectral response 54 of the prior art local oscillator shown in FIG. 5 and compares the spectral response 54 to the maximum desired harmonic noise level, line 52. As can be seen, the prior art oscillator consistently exceeds level 52, especially at the high frequencies that fall into cellular telephone frequency ranges. One known method to reduce the harmonic emissions of FET oscillators is to provide a diode across the gate of the oscillator to clamp the gate.